Many image printing apparatuses have recently been used, and high-speed printing, high resolution, high image quality, and low noise are demanded of these image printing apparatuses.
One image printing apparatus which meets these demands is an ink-jet printer.
The ink-jet printer discharges droplets of a printing solution (ink) from the orifices of a printhead, and adheres the droplets to a printing medium to print an image. The ink-jet printer can achieve non-contact printing and obtain a stable printed image.
Most of ink-jet printers employ a driving method of discharging ink from a plurality of nozzles within a short time in order to print a line in the direction of an ink discharge nozzle line as linear as possible.
However, in this driving method, the number of nozzles to be simultaneously driven increases as the number of nozzles is increased to print a high-resolution image at a high speed. This causes the voltage drop of a nozzle driving power supply voltage, or temporarily increases the negative pressure level in a liquid chamber common to ink tanks, failing to refill the chamber with ink.
To prevent this, nozzles are grouped into several blocks, and the blocks are driven with a delay (by time division), instead of simultaneously driving all the nozzles.
This time division driving method is devised in various ways.
For example, a line formed by discharge is adjusted to a straight line by such adjustment that nozzle positions and the alignment direction of a nozzle array are inclined.
As for a nozzle driving signal, a driving method using a single pulse of one rectangular wave has initially been used. However, this method cannot realize a desired discharge amount, discharge speed, refill frequency, and the like in printing an image at a high speed and high resolution. Thus, a driving method of supplying a plurality of rectangular waves for discharge of one ink droplet is being used.
For example, a thermal ink-jet method of heating a heater, and bubbling and discharging ink generally adopts a double-pulse driving method using two rectangular waves, as shown in FIG. 5.
In the double-pulse driving method, ink on the heater is preheated by a first pulse P1 as a pre-pulse. After an idle time P2, ink is heated, bubbled, and discharged by a second pulse P3 as a main pulse. The ink discharge efficiency is higher, compared to a single pulse using only the second pulse P3 as a main pulse.
The double pulses can control the ink discharge amount and discharge speed by changing the period of the pre-pulse P1 and the idle time P2 of the second pulse.
A printhead used in the ink-jet printer generates bubbles in ink by using heat energy, and discharges ink on the basis of the generation of bubbles. When nozzles are repetitively used within a short time for high-speed, high-resolution image printing, heat energy generated in the printhead is not completely consumed by ink discharge, and some of the heat energy is accumulated as heat. The heat raises the temperature of the printhead, adversely affecting its printing characteristics.
For example, a rise in printhead temperature decreases the viscosity of a printing solution (ink) filled in the printhead and increases the fluidity. The printhead discharges a larger amount of ink than a predetermined discharge amount.
The ink discharge amount larger than the predetermined discharge amount adversely affects the quality of an image to be printed, and increases the ink use amount, resulting in high running cost. Further, excessively heating the printhead may damage the printhead.
To avoid this, a heat dissipation member is attached to the ink-jet printer main body or printhead, or a cooling time for cooling the printhead to a predetermined temperature is set.
To stabilize the ink discharge amount even upon a rise in printhead temperature, driving pulses are controlled in accordance with the printhead temperature, as disclosed in Japanese Patent Laid-Open No. 5-31905.
The printhead is generally operated by double-pulse driving, but when the temperature rises, driving pulses are controlled to a single pulse. This can decrease the discharge efficiency with respect to heat energy, and suppress the discharge amount. Further, as disclosed in Japanese Patent Laid-Open No. 11-170500, printing data is decimated upon a rise in temperature.
In recent years, the number of nozzles increases to several hundred or several thousand in order to meet demands for higher-speed printing and higher resolution. High-speed driving at a driving frequency of several ten kHz is required.
In the conventional driving method, the number of elements to be simultaneously driven every block by time division increases. As a result, the instantaneous maximum current increases, and the voltage drop of the power supply voltage at the intermediate wiring increases.
The number of elements to be simultaneously driven changes depending on printing data. For example, if the number of elements to be simultaneously driven increases in accordance with printing data, a power supply voltage necessary to discharge ink is not applied to the heater, failing to discharge ink.
As a method of solving this problem, the wiring resistance is minimized, a margin for a maximum voltage drop is set, and the set voltage is increased.
However, the method of increasing the set voltage cannot cope with an increase in the number of nozzles and an increase in speed in order to realize higher-speed printing and higher resolution because the breakdown voltage of driving elements is limited.
If the number of elements to be simultaneously driven decreases in accordance with printing data, excessive energy is applied to the heater, decreasing the thermal efficiency and greatly degrading the durability of the heater for heating a driving element.
A method of solving this problem is to count the number of elements to be simultaneously driven in accordance with printing data, and to control the driving pulse and driving voltage, as disclosed in Japanese Patent Laid-Open No. 9-11504.
According to this method, elements to be simultaneously driven are counted, a power loss corresponding to a voltage drop is calculated, and the driving pulse and driving voltage are controlled to compensate for the above-mentioned nozzles which do not discharge ink. This method sets a proper driving pulse and driving voltage calculated by the number of elements to be simultaneously driven in accordance with printing data. Hence, this method is very effective in terms of the thermal efficiency of heating a driving element and the heater durability.
In the high-speed printing method of increasing the number of elements to be simultaneously driven and controlling the high-speed driving pulse, the driving pulse control width must be set large for the purpose of increasing the ink temperature to use efficient double-pulse driving or reducing an increase in voltage drop caused by the wiring resistance. Even if the conventional time division driving method is simply applied to a driving method used for a larger number of nozzles or high-speed driving, a pulse width necessary for a block time required by high-speed printing cannot be ensured.
For example, elements to be simultaneously driven in accordance with printing data are driven at 15 kHz. In addition, the elements to be simultaneously driven are grouped into 16 blocks and driven. In this case, the pulse width ensuring region for driving elements for one block must be set to 3.7 μsec or less.
However, inserting optimal double-pulse driving in the 3.7-μsec width cannot be physically achieved because of the following reason.
That is, the above-described pre-pulse P1 and idle time P2 have given time durations, which enable control operations of increasing the ink discharge amount, and when the printhead temperature rises, decreasing the printhead temperature.
From this, for a small pulse ensuring region where the control becomes impossible, the double-pulse idle time P2 is shortened though this is not an optimal control method.
Japanese Patent Laid-Open No. 7-96608 discloses a method of inserting the pre-pulse P1 into the idle time P2 of the previous block to ensure the idle time P2.
In this method, the idle time must be set to the main pulse P3 or more, and the degree of freedom for control of the discharge amount by the idle time P2 is low.
In addition, blocks are frequently switched. To perform time division by a block signal or the like, a high-speed, high-reliability logic response characteristic is required. This is disadvantageous for a large time division number.
There is also a means for decreasing the time division number. However, the time division number is difficult to change when an output from a carriage encoder is directly used as the driving division number because of an excessive voltage drop and high speed.
Japanese Patent Laid-Open No. 11-170500 discloses a control method of decimating data. This method requires a long data processing time and is disadvantageous in high-speed operation. Simply decimating data results in data loss, degrading the printing quality.